ZFRetryDownloader
一个基于NSURLSession实现的下载模块，并封装了一个下载失败重试的逻辑。
Getting Started
These instructions will get you a copy of the project up and running on your local machine for development and testing purposes. See deployment for notes on how to deploy the project on a live system.
Xcode 9.0 or later; iOS 9.0 SDK or later
Installing
首先从GitHub上克隆项目
git clone https://github.com/zhonglaoban/ZFRetryDownloader.git
然后打开ZFRetryDownloader.xcodeproj，就可以运行啦。
Running the project
运行项目你会看到这个样子
- 图1:下载失败后无法显示图片
- 图2:下载失败会根据设置的数组重试或者下载其他资源
Code Analysis
这个下载器是基于NSURLSession实现的，简单的说一下实现原理。
单例
在面向对象编程中，我们的下载器不需要频繁的创建和销毁，我们只需要管理好每一个下载的Task即可，所以我们用单例实现这个下载器。
注意：在oc中，init是初始化类的属性，alloc才是分配内存空间，所以我们需要对allocWithZone、init都做处理，确保是一个实例。
+ (instancetype)shared {
return [[ZFDownloader alloc] init];
}
- (instancetype)init {
if (instance == nil) {
dispatch_once(&onceToken, ^{
instance = [super init];
});
}
return instance;
}
+ (instancetype)allocWithZone:(struct _NSZone *)zone {
if (instance == nil) {
dispatch_once(&onceToken, ^{
instance = [super allocWithZone:zone];
});
}
return instance;
}
我们创建一个task，然后开始下载。一个task就是一个下载任务，我们需要对不同的任务进行处理，所以我们创建一个ZFTaskDelegate类来处理这些下载任务。
ZFTaskDelegate *delegate = [[ZFTaskDelegate alloc] init];
NSURLSession *session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:delegate delegateQueue:[NSOperationQueue mainQueue]];
NSURLSessionDownloadTask *task = [session downloadTaskWithURL:[NSURL URLWithString:url]];
[task resume];
将一些回调传递给delegate，通知调用者。
delegate.recievedProgressBlock = progressBlock;
delegate.downloadSuccessfulBlock = success;
delegate.downloadFailedBlock = failure;
delegate.savePath = savedPath;
接下来处理delegate中的事情，由于NSURLSessionDownloadDelegate只有下载完成、下载进度的回调，我们的代理还需要实现NSURLSessionTaskDelegate的协议，来处理异常的情况。
下载完成
下载完成后，文件默认存放在tmp文件夹中，会被系统清除，所以我们需要把文件拷贝到其他目录中。
- (void)URLSession:(nonnull NSURLSession *)session downloadTask:(nonnull NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(nonnull NSURL *)location {
NSFileManager *fileManager = [NSFileManager defaultManager];
if ([fileManager fileExistsAtPath:self.savePath] == NO){
if (self.downloadFailedBlock) {
NSError *pathError = [NSError errorWithDomain:NSCocoaErrorDomain code: -1 userInfo:@{NSLocalizedFailureReasonErrorKey: @"目标路径不存在"}];
self.downloadFailedBlock(pathError);
}
return;
}
NSURL *destUrl = [NSURL fileURLWithPath:[self.savePath stringByAppendingPathComponent:downloadTask.response.suggestedFilename]];
NSError *fileMoveError;
if ([fileManager fileExistsAtPath:destUrl.path]){
if (self.downloadSuccessfulBlock) {
self.downloadSuccessfulBlock(destUrl);
}
return;
}
BOOL result = [fileManager moveItemAtURL:location toURL:destUrl error:&fileMoveError];
if (result) {
if (self.downloadSuccessfulBlock) {
self.downloadSuccessfulBlock(destUrl);
}
}else {
if (self.downloadFailedBlock) {
self.downloadFailedBlock(fileMoveError);
}
}
}
下载进度计算
下载进度计算是根据当前下载数据的大小比上下载数据的大小
-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
float progress = (float)totalBytesWritten / totalBytesExpectedToWrite;
if (self.recievedProgressBlock) {
self.recievedProgressBlock(progress);
}
}
错误处理
当下载出错时会走didCompleteWithError这个方法。
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
if (error) {
if (self.downloadFailedBlock) {
self.downloadFailedBlock(error);
}
}
}
下载重试逻辑
下载失败后我们希望自动重试，或者去下载其他资源，比如A->A->B、A->A->A->C等这样的逻辑。我的思路是把这些任务放在一个队列中，一次执行一个任务，如果有一个任务成功了，后面的就不执行了，这里我用的是GCD的信号量来控制的。
- (void)retryDownloadFileWithUrls:(NSArray *)urls savedPath:(NSString* _Nonnull)savedPath progress:(void (^_Nullable)(float progress))progressBlock success:(void (^ _Nullable )(NSURL * _Nonnull location))successBlock failure:(void (^ _Nonnull )(NSError * _Nonnull error))failureBlock {
__block BOOL shouldRetry = YES;
dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
dispatch_async(dispatch_queue_create("com.zf.retryDownloadFileWithUrls", DISPATCH_QUEUE_SERIAL), ^{
for (NSString *url in urls) {
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
if (shouldRetry == NO) {
dispatch_semaphore_signal(semaphore);
return;
}
[self downloadFileWithUrl:url savedPath:savedPath progress:^(float progress) {
if (progressBlock) {
progressBlock(progress);
}
} success:^(NSURL * _Nonnull location) {
shouldRetry = NO;
dispatch_semaphore_signal(semaphore);
if (successBlock) {
successBlock(location);
}
} failure:^(NSError * _Nonnull error) {
dispatch_semaphore_signal(semaphore);
if (failureBlock) {
failureBlock(error);
}
}];
}
});
}
How to use
使用普通下载
- (IBAction)normalDownload:(id)sender {
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *savedPath = [paths objectAtIndex:0];
[[ZFDownloader shared] downloadFileWithUrl:[self.retryUrls firstObject] savedPath:savedPath progress:^(float progress) {
NSLog(@"%f", progress);
} success:^(NSURL * _Nonnull location) {
self.imageView.image = [UIImage imageWithContentsOfFile:location.path];
} failure:^(NSError * _Nonnull error) {
NSLog(@"%@", error);
}];
}
使用重试下载
- (IBAction)retryDownload:(id)sender {
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *savedPath = [paths objectAtIndex:0];
[[ZFDownloader shared] retryDownloadFileWithUrls:self.retryUrls savedPath:savedPath progress:^(float progress) {
NSLog(@"%f", progress);
} success:^(NSURL * _Nonnull location) {
self.imageView.image = [UIImage imageWithContentsOfFile:location.path];
} failure:^(NSError * _Nonnull error) {
NSLog(@"%@", error);
}];
}
github地址
Authors
- zhonglaoban - Initial work - zhonglaoban
License
This project is licensed under the MIT License - see the LICENSE.md file for details